Human vaccines, including those for influenza, are traditionally grown in embryonated chicken eggs. Advances in influenza reverse genetics lead to the possibility of using cell culture systems to produce influenza vaccines; however, the replicative complex used in current vaccines does not grow well in this system. Researchers at St. Jude developed a modification of the genetic makeup of the influenza virus backbone that results in a replicative complex that does grow well in cell culture, particularly in Vero cell culture. Influenza vaccine strains made with this modified viral backbone have increased yield and speed of growth in cell culture, which are very desirable traits in vaccine production. Application of this invention helps to make the transfer of traditional vaccine production from eggs to cell culture systems feasible.